A lean-burn engine may be supplied with a lean mixture of air and fuel (oxygen-rich mixture) as a means to improve vehicle fuel economy. The engine-out exhaust emitted from such engines during periods of lean-burn operation may include a relatively high content of oxygen (O2), a relatively low content of carbon monoxide (CO) and unburned/partially-burned hydrocarbons (hereafter HC's), and small amounts of nitrogen oxides primarily comprised of NO and NO2 (collectively referred to as NOX gases). The concentration of several of these gaseous emissions, however, may need to be decreased to the greatest extent feasible before the engine-out exhaust is expelled to the atmosphere from the vehicle's tailpipe. To this end, an exhaust aftertreatment system may be installed downstream of the vehicle engine to control and reduce the various unwanted emissions and particulate matter contained in the engine-out exhaust. A typical exhaust aftertreatment system usually aspires to (1) oxidize CO into carbon dioxide (CO2), (2) oxidize HC's into CO2 and water (H2O), (3) convert NOX gases into nitrogen (N2) and O2, and remove any other unwanted matter.
Traditional catalytic converters outfitted with a three-way catalyst (TWC) have been implemented in many exhaust aftertreatment system designs. The TWO generally includes some combination of platinum group metals (PGM) that can simultaneously oxidize CO and HC's and reduce NOX gases. Catalytic converters of this kind have been known to function quite effectively when the engine operates with a stoichiometric mixture of air and fuel. This is because the engine-out exhaust generated from the combustion of a stoichiometric air/fuel mixture generally includes an appropriate balance of reductants (CO, HC's, and H2) and oxidants (O2) to concurrently reduce the NOX gases and oxidize any CO and NC's through various coupled catalytic reactions. But TWC-equipped catalytic converters are generally not able to efficiently reduce NOX gases when the engine operates with a lean mixture of air and fuel. The low levels of reductants and the high O2 content in the engine-out exhaust make such a reaction kinetically unfavorable in most instances.
A lean NOX trap, or LNT, is but one available option that may be employed in the exhaust aftertreatment system to help remove NOX gases contained in the engine-out exhaust of a lean-burn engine. A LNT generally operates by feeding the engine-out exhaust expelled from the lean-burn engine across and/or through an LNT catalyst material that exhibits NOX gas trapping and conversion capabilities. The LNT catalyst material oxidizes NO to NO2 and simultaneously traps or “stores” NO2 as a nitrate species when the lean-burn engine is combusting a lean mixture of air and fuel. The efficiency of NOX gas removal and storage may be enhanced, in some instances, by increasing the proportion of NO2 in the total NOX emission so as to reduce the oxidative demand (NO to NO2) on the LNT catalyst material. The NOX storage capacity of the LNT catalyst material, however, is not unlimited and at some point may need to be regenerated or purged of the NOX-derived nitrate compounds. The LNT catalyst material may be regenerated by momentarily switching the mixture of air and fuel supplied to the lean-burn engine from lean to rich. The resultant delivery of a rich-burn engine-out exhaust to the LNT catalyst material causes the NOX-derived nitrate compounds to become thermodynamically unstable which, in turn, triggers the release of NOX gases and regenerates future NOX storage sites. The liberated NOX gases are then reduced, largely to N2, by the excess reductants—such as CO, HC's and/or H2— present in the rich-burn engine effluents. The overall conversion efficiency of some LNTs, aided by the appropriate cycling between a lean and rich mixture of air and fuel, have been shown to remove more than 90% of NOX gases contained in the engine-out exhaust of lean-burn engines over lengthy periods of time.
A conventional LNT typically includes a canister with an inlet that receives the engine-out exhaust emitted from the lean-burn engine and an outlet that delivers the engine-out exhaust from the canister. The canister may house a support body that communicates the engine-out exhaust from the inlet to the outlet over a catalyst material. The catalyst material is typically a mixture of PGMs and an alkali or alkaline earth metal compound dispersed within a high surface-area washcoat. The mixture of PGMs includes platinum, which catalyzes the oxidation of NO and to some extent the reduction of NOX gases, and rhodium, which primarily catalyzes the reduction of NOX gases. The alkali or alkaline earth metal compound provides trap sites for the reversible storage of NO2 as a metal nitrate. Of these various materials dispersed in the washcoat, platinum is usually present in the greatest amount. One specific LNT catalyst material known to skilled artisans includes an alumina washcoat appropriately loaded with platinum, rhodium, and barium oxide. But the use of platinum group metals, especially the relatively large amounts of platinum, in conventional LNT catalyst materials is rather expensive. Platinum has also shown a tendency to lose some catalytic activity when exposed to engine-out exhaust at higher operating temperatures.
The incorporation of a lean NOX trap into an exhaust aftertreatment system for a lean-burn engine is thus an attractive, yet challenging, option for removing unwanted emissions including NOX gases from the engine-out exhaust. Such technology is constantly in need of innovative developments and contributions that can help advance to this and other related fields of technological art.